Prior two-way radio communication systems have been concerned with producing as large a dynamic range as practical to provide communication over a large area. The present invention discloses a method whereby a plurality of portable units can operate within a few feet of each other, all within a predetermined dimensioned enclosed area, and communicate simultaneously each with its own base unit on different frequencies in the same band without harmful interference. When interference occurs, an individual has difficulty in communicating clearly with someone on the other end of a radio link because other radio signals somehow jam or interfere with the conversation in progress. If this interference precludes normal conversation from occurring, it is called harmful interference. Occasional static, strange sounds or voices being heard by the users that does not affect a conversation in progress is not considered harmful interference. If the communication link is for data transmission, interference would be considered harmful if transmission errors occur that could not be corrected by digital techniques.
Prior art in this specific area is truly limited. As disclosed in U.S. Pat. No. 4,152,648 by Delogne, a radio-communication system for confined spaces merely discloses a radiating transmission line. U.S. Pat. No. 4,165,487 describes a low power communication system that provides for only one-way transmission and uses only one frequency. Indeed, there are situations where many discrete communication channels need to be operating simultaneously in a limited area without interfering with each other. Prior communication systems operate portable transceivers at output power levels of +30 dBm or more and receive sensitivities at a minimum of -115 dBm. This results in a dynamic range in excess of 145 dB. This wide dynamic range is very useful where maximum area coverage is desired. However, a wide dynamic range in a confined area, where portable units are relatively close to base antennas and to each other, will cause harmful interference to other units operating in the system. If the variations in signal strengths between the portable units and base units are minimized as the portable moves about the confined area, then a system could be designed such that any spurious signals generated would be near or below the threshold level of sensitivity of the receivers in the system.
If several two-way radios operate in close physical proximity on different frequencies in the same band, receiver desensitization and spurious signals such as intermodulation (IM) products result in interference. A very severe interference problem results when third order intermodulation products are generated when the mixing of radio signals occur. The offending signals on frequencies F.sub.1 and F.sub.2 are generated according to the well known equations: EQU 2A-B=F.sub.1 ( 1) EQU A+B-C=F.sub.2 ( 2)
where:
A=Tx freq 1 PA1 B=Tx freq 2 PA1 C=Tx freq 3 PA1 F.sub.1, F.sub.2 =Intermodulation (IM) product frequencies
Where frequency F.sub.1 or F.sub.2 coincides with another users frequency, interference occurs. Fifth and seventh order IM products are also generated but usually their energy levels are so low that they do not cause harmful interference.
Intermodulation products become significant when two strong radio frequency (RF) fields exist at the same point and mixing takes place. One of the places this mixing can occur is in the transmitter output stages of each unit transmitting. This is called high level mixing. In the case of several portable radios operating within a few feet of each other and two or more are transmitting simultaneously, RF energy radiated from the antenna of each unit is received by the other transmitting antennas and is coupled back to each transmitter's output amplifying stage. This results in a mixing of the signals whereby intermodulation products are formed and re-radiated by the transmitting antenna.
Mixing can also occur in the front-end stages of a radio receiver. When two strong signals are present at the input of a receiver, mixing can take place and IM products are internally generated. This is called receiver IM.
Careful selection, then, of the portable transmitter frequencies is imperative. No IM product should form under any combination of portable transmit frequencies which coincide with other portable receiver frequencies. Even if variations in the signal strength from portable to base are minimized, weak signals generated spuriously in the portable's transmitter output stages will be received by other portables operating close by in the receive mode. Further, front-end mixing will invariable take place due to the close proximity of other portables transmitting. IM products in the portable transmit frequency range are unavoidable; however, they can be minimized so that these spurious signals do not interfere with bona fida signals received by any base unit receiver.
Another very severe interference problem is receiver desensitization. Receivers are desensitized when a strong RF signal in the same band as the receiver appears at the receiver input. If the signal is on the frequency the receiver is tuned to, then no problems exists. However, if the signal is not on the receiver frequency, the receiver will be desensitized and be hampered from receiving weaker signals coming in that are on the receiver frequency. Without protection from strong signals, harmful interference will result when a plurality of portable radio units operate in close proximity using prior art techniques.